This invention relates to the forming of metal articles, specifically by the process known as superplastic forming.
Certain metallic alloys exhibit the property, in a certain temperature range and when deformed within a certain range of strain rates, of behaving like a viscous fluid and undergoing large deformations without necking or fracture. This property is normally exploited by applying differential gas pressure to either side of a sheet of superplastically formable material to blow the sheet to the form of a mold tool in which the sheet is placed. Such alloys, when pressed together with sufficient pressure within said temperature range, are also capable of joining together permanently under said pressure. This process is called diffusion bonding.
Hollow stiffened structures may be readily made using two or more sheets of superplastically formable material sealably connected together to form one or more gas-tight envelopes and expanded by pressure of inert gas within the envelope(s) to the shape of an enveloping mold tool.